Concave extrusion of mesh
One way...


Create an empty circle
Give it a Mirror modifier with Bisect checked in the mirror axis. Move half the vertices toward the center, until you get the shape you like, and apply the modifier.
With all selected in Edit mode, CtrlF Face menu &gt; Grid Fill.
Assign a Solidify modifier
Assign a Bevel modifier, by angle, (Segments 2, Profile 1?) to catch the sharp edges
Assign a Subdivision Surface modifier

Now select the central vertex, and under O, proportional editing, raise it up a bit, with a diameter and falloff profile you like, to get the curve.
..and maybe, in Edit, CtrlV Vertex menu, &gt; Smooth the vertices, a touch

Grid Fill needs an even number of vertices to work cleanly, the Mirror modifier helps ensure that, even if your petal isn't quite the same shape as this one.